I'll give you some general info not specific to the Ram. Start by checking the coolant level. If thats good, check the thermostat. You could do that by starting your truck while the engine is cold and touching the upper (I think) radiator hose. It should stay cold until your engine reaches operating temp and then the hose should rapidly get hot. That signifies that your thermostat has opened. It still could be your thermostat since it opens and closes based on the temperature of the coolant and may be getting stuck wide open. To be honest, if you haven't changed it in years, go ahead and change it. If the coolant is fine, and the thermostat is good, I'd check for a blockage in the heater core.